In the Hall of the Mountain King: Champions Mini-Expansion

The noble effort to reclaim the mountain attracts heroes of trolldom. Each player gains a legendary troll champion which grants powerful benefits!

(First available in the Deluxe Edition of the game for FREE, and later sold separately by the publisher.)

Each game, 1 champion will remain undrafted. This champion will gain 1 item to become more enticing next time. When you draft a champion, you get all of the items it has accumulated while "sitting on the bench".

From the rule card: Champions can be used in Competitive play, Team play, and are compatible with Co-op and Solo play.